It’s difficult to explain exactly how it happened, but 386 New York City shelter cats have contracted a rare avian flu that had never before been seen in cats, and had not been seen in any animals at all in over 10 years.

In November, a gray-striped kitten at a Bronx animal shelter named Alfred came down with the flu, a strain called H7N2 last seen in 2006 at poultry markets in New York and elsewhere. He died on November 12, and the virus quickly spread throughout the city’s shelter population.

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Now, The New York Times reports, the entire feline population of New York City’s shelter system—some 500 cats—is being housed at a temporary quarantine center in Queens. The Times described it as “something out of a post-apocalyptic cat video.”

The virus is a mild form of H7N2 and is usually not life-threatening to cats. According to the CDC, it is only rarely transmissible to humans and causes little more than slight illness. Only one person has tested positive for the virus so far in the current outbreak, a veterinarian who was working with the sick cats.

But the mystery of how a strain of avian flu suddenly infected such a large population of New York City’s cats is still cause for concern.

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“Any time influenza viruses start to behave in an unusual way, there’s a concern about what might happen,” Aleisha Swartz, a doctor from the University of Wisconsin veterinary school, told The New York Times. “There’s this virus that popped up, and if we didn’t respond, it could have become widespread in cats all over the place.”

For cats, the main symptoms are a runny nose, congestion, coughing and lip smacking. The cats can leave once they are determined to be flu free, which could take weeks.